1. Field of the Invention
The present invention relates to a technique to prevent damage on a sensor element by thermal shock in an air/fuel ratio sensor which is used for detecting an air/fuel ratio in air/fuel ratio feedback control for an internal combustion engine and is provided with a heater for promoting activation of the sensor element.
2. Related Art of the Invention
There is an electronically controlled fuel injection device for an internal combustion engine which is so constituted that an air/fuel ratio of an engine intake mixture is detected based on a concentration of a specific component such as oxygen in exhaust gas and feedback control of a fuel injection amount is carried out so that the air/fuel ratio gets close to a theocratical air/fuel ratio (See The Japanese Unexamined Patent Publication No. 60-240840 etc.).
As an air/fuel ratio sensor used for the above air/fuel ratio feedback control, there have been used such a sensor that electrodes are formed each on inner and outer surfaces of a zirconia (oxygen ion conductive solid electrolyte) tube as a sensor element, an electromotive force is generated between the above electrodes corresponding to a ratio between an oxygen concentration (reference oxygen concentration) in the air introduced into the inside of the tube and the oxygen concentration in the exhaust gas on the outside, and by monitoring this electromotive force, not only the oxygen concentration in the exhaust gas but also rich/lean against the theoretical air/fuel ratio in the engine intake mixture is detected (See The Japanese Unexamined Utility Model Publication No. 63-51273, etc.), and another sensor that the theoretical air/fuel ratio is detected using a change in a resistance value caused by the oxygen concentration (oxygen partial pressure) of a transition metal oxide such as titania as a sensor element.
Also, as the above sensor element is not fully activated at a low temperature and can not favorably detect the air/fuel ratio, there are those with a built-in electric heater for forcibly heating the sensor element to promote activation before warming up when an exhaust temperature is low.
The above sensor element is usually provided at a collection part of an exhaust manifold, but there are those which have another air/fuel ratio sensor in the same structure at the lower stream side of a CCRO for exhaust purification provided in an exhaust system other than the above air/fuel ratio sensor and carry out feedback control of the air/fuel ratio using those two air/fuel ratio sensors (See The Japanese Unexamined Patent Publication No. 58-72647, etc.).
As shown in FIG. 7 and FIG. 8 when an engine is started after cooling down, an exhaust temperature is raised and then, temperature of a catalyzer 1 and an exhaust pipe 2 are raised with some delay.
Water content (H.sub.2 O) is generally included in the exhaust as vapor, and as shown in FIG. 7, the exhaust is cooled in the exhaust pipe 2 provided away from the exhaust manifold 3 to below a dew-point temperature and moisture is generated and adheres to the surface.
Particularly as reaction of unburnt gas is promoted by the catalyzer, an amount of moisture included in the exhaust is large at the lower stream side of the catalyzer 1, and in the air/fuel ratio feedback control system provided with oxygen sensors 4a and 4b each at an upper and a lower stream sides of the CCRO, a large amount of water is generated around an element of the oxygen sensor 4b mounted on the lower stream side of the catalyzer 1.
Therefore, in the oxygen sensor 4b on the lower stream side of the catalyzer 1, a large amount of moisture in the exhaust adheres to the sensor element (zirconia tube, etc.) with starting of the engine, and when the sensor element is heated by turning on the above heater with the moisture left on the element, the moisture evaporates from the sensor element surface. Then, it might cause a problem that a difference in the temperature between inside and outside of the sensor element such as zirconia and titania is widened and the ceramic element of the above oxygen sensor 4b is split by thermal shock.